Distributed feedback semiconductor lasers (hereinafter, referred to as “DFB lasers”) are commonly used as light sources for long distance and large capacity optical data communication along optical fiber links. In addition, single mode DFBs have well-defined rise and fall times of the single optical mode as may be required for high data rate applications.
Many approaches for controlling the mode spectrum of DFB lasers have been developed in recent years. For example, index coupled devices which include periodic variations of the refractive index in the cavity length direction have been commonly employed to provide single longitudinal mode oscillation at a wavelength corresponding to the period of change of the refractive index. However, index-coupled DFB lasers tend to produce a pair of longitudinal oscillation modes having equal threshold gain which often results in multi-mode operation.
Mode degeneracy in index coupled lasers has been controlled to some extent with facet coatings applied to each end face of the laser to provide asymmetric reflectivity for the oscillation modes. Facet coatings, however, may not ensure single mode operation because of the random facet phases introduced by end face cleaving. Therefore, even with such asymmetric coatings, the probability that an index-coupled DFB laser device will oscillate in a desired single longitudinal mode is only about 50–70%.
In contrast, complex-coupled DFBs, which periodically vary the gain in the cavity length direction, oscillate in a single mode having a reduced threshold gain. In conventional gain-coupled semiconductor DFB lasers, light is fed back by a perturbation in the gain or loss coefficient provided by a diffraction grating in addition to the index perturbation.
The grating is typically generated by etching grooves through a multi-quantum well active region and re-growing a buffer material in the etched grooves. The grating in conventional devices is not formed from high reactivity materials, such as for example, aluminum. High reactivity materials essentially oxidize immediately when exposed to air after being etched complicating the regrowth process. Therefore, in conventional devices, the multi-quantum well active region that is etched to form a grating is commonly formed from low reactivity materials, such as, for example, InGaAs/InGaAsP or the like which do not oxidize when etched so that re-growth over the grating may be more readily accomplished.
However, commonly used, low reactivity material systems typically have relatively small conduction band offsets with relatively poor carrier confinement which may create a significant thermally activated electron leakage current. The leakage current may result in higher laser threshold currents and poor system performance as a function of temperature.